To explore the novel combination of sirolimus and mycophenolate mofetil (MMF) as graft-vs-host disease (GvHD) prevention in human leukocyte antigen (HLA)-matched related donor peripheral blood stem cell (PBSC) or marrow transplantation (BMT), collectively hematopoietic stem cell transplantation (HSCT). This study will report the toxicities associated with this drug combination.